The present invention relates to a multi-color thermal transfer printing apparatus that employs a plurality of separate thermal print heads and particularly concerns a generally arcuate arrangement for the print heads whereby registration errors may be minimized. The present invention also particularly concerns a pressure adjuster for use on a thermal print head in order to adjust the print pressure.
Typical thermal print heads currently available generally comprise a flat ceramic substrate that is provided on one side with a plurality of thermal elements and the necessary electronic circuitry for controlling the activation of the thermal elements. The electronic circuitry is typically covered by a shield for protecting the circuitry from foreign particles, moisture, and other damaging contact. Although the configuration of the electronics shield may vary from print head to print head, the shield normally protrudes substantially from the surface of the ceramic substrate. Mounted on the opposite side of the ceramic substrate, typically, is an aluminum heat sink for providing cooling.
In operation, the thermal print head is usually used in conjunction with a platen and an ink transfer foil that carries a thermal transfer ink. The substrate to be printed and the ink foil are presented to the print head between the thermal elements of the print head and the platen such that the ink foil is adjacent to the print head and the substrate is adjacent to the platen. The print head is then biased against the platen, and selected thermal elements are heated to effect a transfer of the ink from the ink foil to the substrate surface.
With a typical thermal print head, the substrate to be printed and the ink foil must be introduced to the print head at an angle sufficient to clear the on-board print head electronics and electronics shield. If more than one print head is used for a particular printing job and the print heads are arranged in a straight line, a roller must follow each print head in order to ensure that the substrate enters the printing area of the next print head at the proper angle to clear the electronics. However, each roller that the web must wrap around introduces some error that can effect print registration. Where multiple print heads are in use, proper registration between print heads as well as between the substrate to be printed and each print head is necessary in order to avoid color overlap or other printing errors relating to the improper positioning of the ink on the sheet.
In addition to proper registration, the proper pressure and thermal energy must be applied by the print head to the substrate and ink foil in order to produce a good quality print. Different widths of the substrate and/or foil can affect the amount of print pressure required for satisfactory printing. For example, wider substrate and foil widths require additional print head pressure, whereas narrower widths require less pressure. If the same print head is to be used for substrates and ink foils of different widths, the print head pressure must be adjusted whenever the substrate or ink foil width is changed.